Optical mass data storage systems have developed to the point where they are finding increased application to information processing systems. The advantages of the optical techniques lie in their packing density and the increased data rates with random access retrieval. The potential for improvement in optical data storage will most likely be directed toward the record medium. Although acceptable archival materials for permanent data storage have been developed, research continues to find more efficient techniques and materials to increase packing density, signal-to-noise ratio, and data rates. Nevertheless, in order for optical storage systems to provide total competition for magnetic storage systems, it will be necessary to provide an efficient erasable and reusable medium, wherein the erasing procedure does not require an unacceptably long execution time which would preclude its use in an information processing system.
Information can be recorded by exposure of a portion of an optical recording medium to a recording light beam, thereby changing the local optical properties of the exposed portion. The simplest such recording medium is a layer of a light absorptive material overlying a substrate wherein information is recorded by locally melting or ablating the absorptive layer to form a pit therein. The presence of the pit results in a change in the local transmission and/or the reflectivity of the recording medium, one of which is detected during readout of this information.
U.S. Pat. No. 4,097,895, entitled "MULTILAYERED OPTICAL RECORD", issued June 27, 1978, to F. W. Spong, disclosed an optical recording medium which comprises a light reflective layer coated with a light absorptive layer, wherein the thickness of the absorptive layer is chosen so that the reflectivity of the recording medium is reduced. U.S. Pat. No. 4,216,501, entitled "OPTICAL ANTIREFLECTIVE INFORMATION RECORD", issued Aug. 5, 1980, to A. E. Bell, disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorptive layers of the Spong optical recording medium. This medium permits the use of a broader class of materials and a lower reflectivity recording medium or information record than that provided by the recording medium disclosed by the Spong patent.
U.S. patent application, Ser. No. 254,649, filed Apr. 16, 1981, for F. W. Spong and A. E. Bell, which is a continuation-in-part of U.S. patent application, Ser. No. 174,844, entitled "INFORMATION RECORD AND A METHOD OF REVERSIBLY RECORDING AND ERASING INFORMATION THEREON", filed Aug. 4, 1980, and now abandoned, disclosed an information record in which information can be recorded, erased and rerecorded, and which includes a capping layer overlying the absorptive layer. The capping layer inhibits, up to a maximum power, irreversible recording such as the formation of a pit in the absorptive layer, upon exposure to an information recording or information erasing light beam.
In U.S. Pat. No. 4,425,570, entitled "REVERSIBLE RECORDING MEDIUM AND INFORMATION RECORD", issued Jan. 10, 1984, to A. E. Bell and Y. Arie, there is disclosed an improved recording medium comprising an absorptive layer overlying a substrate which comprises one or more domains of a light absorbing material, which can be reversibly switched from a first state to a second state having different optical properties, embedded in a matrix effective for inhibiting an irreversible change in the optical properties of the domain material. The recording medium of the Bell et al. patent permits a range of recording beam powers which produce a reversible recording which is larger than that which was theretofore known.
Erasable optical recordings are thus made by heating a microscopic spot on the record medium to a level where its optical properties change, short of causing a rupture of the material which would form a permanent pit. The heating process changes the absorptive layer at the heated spot from a crystalline to an amorphous state. Reheating the spot at a lower temperature and for a longer time period anneals this spot, causing the amorphized spot to return to its crystalline state wherein its original optical properties are perfectly recovered.
Although recording and playback of information on an optical disc medium typically occurs as the record track makes a single pass by the optical head at a disc rotational speed of, for example, 30 revolutions per second, the annealing process used for erasure requires exposure to the reduced heat level for a far greater period of time. It can be readily seen that the spot size is of great significance in the erasing process. If the same small spot used for recording is also used for erasing, many revolutions of the disc will be required to complete the annealing process such that the recorded spot has returned entirely to its crystalline state.
Forming the erase beam into an elliptical shape provides two dimensions of improvement in the erasure process. First, the elongated spot provides longer exposure of the recorded area to the lower-energy erase power, thereby decreasing the number of exposures, or revolutions, necessary to effect complete erasure of the recorded area. Second, a slight rotation of the elliptical spot will widen the erase beam path slightly to cover at least the entire width of the track of the recorded spot so that even the edges of the spot receive erase energy on every pass.